System for pre-purification of a feed gas stream

ABSTRACT

A system and method of pre-purification of a feed gas stream is provided that is particularly suitable for pre-purification of a feed air stream in cryogenic air separation unit. The disclosed pre-purification systems and methods are configured to remove substantially all of the hydrogen, carbon monoxide, water, and carbon dioxide impurities from a feed air stream and is particularly suitable for use in a high purity or ultra-high purity nitrogen plant. The pre-purification systems and methods preferably employ two or more separate layers of hopcalite catalyst with the successive layers of the hopcalite separated by a zeolite adsorbent layer that removes water and carbon dioxide produced in the hopcalite layers. Alternatively, the pre-purification systems and methods employ a hopcalite catalyst layer and a noble metal catalyst layer separated by a zeolite adsorbent layer that removes water and carbon dioxide produced in the hopcalite layer.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of International Application No.PCT/US2020/064163, filed on Dec. 10, 2020 which claimed the benefit ofU.S. provisional patent application Ser. No. 63/067,539 filed Aug. 19,2020 the disclosures of which are incorporated by reference.

TECHNICAL FIELD

The present invention relates to a system and method for removingimpurities from a feed gas stream, and more particularly, to a methodand apparatus for removing water, carbon dioxide, hydrogen, and carbonmonoxide from a feed gas stream prior to its introduction into acryogenic distillation system. More specifically, the present inventionrelates to a system and method for pre-purification of a feed air streamin a cryogenic air separation unit.

BACKGROUND

Adsorption is well established technology for the purification of gasesand for the treatment of fluid waste streams. Purification andseparation of atmospheric air comprises one of the main areas in whichadsorption methods are widely used. For an increase of their efficiency,novel and improved pre-purification systems and methods are continuouslybeing developed.

One of the areas of strong commercial and technical interest representspre-purification of air before its cryogenic distillation. Conventionalair separation units for the production of nitrogen (N₂) and oxygen (O₂)and also for argon (Ar) by the cryogenic separation of air are basicallycomprised of two or at least three, respectively, integrateddistillation columns which operate at very low temperatures. Due tothese low temperatures, it is essential that water vapor (H₂O), andcarbon dioxide (CO₂) is removed from the compressed air feed to an airseparation unit to prevent freeze up of components within the airseparation unit.

Current commercial methods for the pre-purification of feed air includetemperature and/or pressure swing adsorption units that employ layers ofadsorbent materials together with optional catalytic pre-purificationtechniques. A pre-purification unit (PPU) situated upstream of thecryogenic distillation system is typically used that includes an upfrontadsorbent layer to remove water, carbon dioxide as well as hydrocarbonsand other contaminants including oxides of nitrogen. Such PPU may alsooptionally include one or more catalysts targeted to remove one or morecontaminants followed by a final adsorbent layer downstream of theoptional catalysts to remove the contaminants produced by the catalysisprocess.

If not removed, water and carbon dioxide present in the feed air willfreeze out and block heat exchangers employed for cooling the feed airprior to distillation in the cryogenic distillation columns. Removal ofhydrocarbons and nitrous oxides is often required to ensure the safeoperation of such cryogenic distillation systems that typically involveprocessing oxygen-rich streams.

Before entering the PPU, atmospheric air is typically compressed to anelevated pressure from about 0.45 MPa to 1.1 MPa, followed by acombination of cooling steps and removal of condensed water. The cooledfeed air stream is then passed to a PPU where any remaining water andcarbon dioxide are first removed by adsorption in a bed of a molecularsieve and/or activated alumina. The air stream exiting the bed ofmolecular sieve and/or activated alumina is substantially free of carbondioxide, water, hydrocarbons, and nitrous oxide. Preferably, to avoidfreeze-out, the content of water in the compressed and pre-purified airfeed stream must be less than 0.1 ppm (part per million) while thecontent of carbon dioxide in the compressed and pre-purified air feedstream must be less than 1.0 ppm. From a safety perspective, thecompressed and pre-purified air should be substantially free of heavyhydrocarbons and nitrous oxides.

In addition, some applications for the electronics industry and selectedother industries require the removal of hydrogen and/or carbon monoxidefrom the feed air stream before processing the feed air stream in thecryogenic distillation system to produce a high purity or ultra-highpurity nitrogen product. A conventional PPU having only a bed ofmolecular sieve and/or activated alumina is quite capable of removingcarbon dioxide, water, hydrocarbons, and nitrous oxide from the cooledfeed air. However, the activated alumina or molecular sieve are noteffective for the substantial removal of carbon monoxide or hydrogenthat may be present in the feed air.

Prior art techniques of removing carbon monoxide and hydrogen in suchapplications have used catalytic based pre-purification techniqueswithin the PPU. For example, pre-purification processes requiringremoval of hydrogen often use a noble metal containing catalyst such asa platinum or palladium containing catalyst material. Likewise, inapplications requiring removal of carbon monoxide or removal of bothcarbon monoxide and hydrogen use of catalytic materials such ashopcalite with or without noble metal containing catalysts. As usedherein, the term ‘hopcalite’ is not used as a tradename but rather isused generically to refer to a catalyst material that comprises amixture of copper oxide and manganese oxide.

For example, U.S. Pat. No. 6,048,509 discloses a method and processutilizing a modified precious metal catalyst (platinum or palladium andat least one member selected from the group consisting of iron, cobalt,nickel, manganese, copper, chromium, tin, lead and cerium on alumina)for oxidation of carbon monoxide to carbon dioxide, followed by waterremoval in an adsorbent layer and carbon dioxide removal in a secondadsorbent layer. An option for hydrogen removal is provided with asecond noble metal containing catalyst layer followed by water removalin subsequent adsorbent layers.

Another example is highlighted in U.S. Pat. No. 6,093,379 whichdiscloses a process for combined hydrogen and carbon monoxide removalconsisting of a first layer to adsorb water and carbon dioxide onalumina or zeolite, and a second layer of a precious metal catalyst(palladium on alumina) to simultaneously oxidize carbon monoxide, adsorbthe formed carbon dioxide and chemisorb hydrogen.

Other prior art references teach the use of other catalyst materialssuch as hopcalite to remove carbon monoxide and hydrogen. Two suchexamples of use of hopcalite for pre-purification to remove hydrogenfrom air are U.S. Patent Application Publication No. 2003/064014 (Kumaret al.) and U.S. Pat. No. 8,940,263 (Golden, et al.) The Kumar et al.reference shows that it has been well known for over 20 years that ahopcalite catalyst removes hydrogen and carbon monoxide from air, and isparticularly useful for removing both carbon monoxide and hydrogen froma feed air stream during pre-purification in cryogenic air separationunits. Golden, et al. also discloses the use of a single layer ofhopcalite for removal of substantially all of the hydrogen and carbonmonoxide. The examples in Golden et al. confirm what is taught in Kumaret al. that hydrogen is chemisorbed in the single layer of hopcalitematerial such that the use of a longer bed of hopcalite catalyst whichtranslates to longer residence times of the dry gas in the hopcalitelayer generally improves the hydrogen chemisorption process in thehopcalite material

While the above-identified prior art pre-purification systems andmethods target removal of impurities such as hydrogen, carbon monoxide,water, and carbon dioxide from feed air streams, the relative costsassociated with pre-purification systems and methods remain high.Accordingly, there is a continuing need to improve such pre-purificationsystems and processes, particularly to reduce the costs of suchpre-purification without sacrificing performance. In other words, thereis a need for improved systems and methods for pre-purification of anincoming feed air stream to a cryogenic air separation unit, includingsubstantial removal of hydrogen, carbon monoxide, water and carbondioxide in the production of high purity or ultra-high purity nitrogenthat has cost advantages and performance advantages over prior artpre-purification systems and methods.

SUMMARY OF THE INVENTION

The present invention may broadly be characterized as a pre-purificationsystem or pre-purifier unit for air-separation plants comprising: (i) avessel having an inlet configured for receiving a stream of compressedfeed air and an outlet configured to direct a purified streamsubstantially free of water, carbon dioxide, carbon monoxide andhydrogen to another part of the air separation unit; (ii) a firstpurification section disposed within the vessel comprising at least onelayer of adsorbent configured to remove water and carbon dioxide fromthe compressed feed air stream and yield a dry feed stream substantiallyfree of water and carbon dioxide; (iii) a second purification sectiondisposed within the vessel downstream of the first purification section,the second purification section having a multi-layer arrangement ofcatalysts and adsorbents to yield an intermediate purified streamsubstantially free of hydrogen and carbon monoxide; and (iv) a thirdpurification section disposed downstream of the second purificationsection and comprising at least one other layer of adsorbent configuredto remove water and carbon dioxide from the intermediate purified streamand yield a purified stream substantially free of water, carbon dioxide,carbon monoxide and hydrogen.

The above-described pre-purification systems may also include one ormore additional layers of manganese oxide and copper oxide containingcatalysts as well as one or more additional adsorbent layers disposedbetween the catalyst layers to remove any water and carbon dioxideexiting the catalyst layers.

BRIEF DESCRIPTION OF THE DRAWINGS

While the specification concludes with one or more claims specificallypointing out the subject matter that Applicants regard as the invention,it is believed that the present systems and methods for pre-purificationof a feed gas stream will be better understood when taken in connectionwith the accompanying drawing in which:

FIG. 1 depicts a partial, cross-section view of a section of apre-purification unit or pre-purification vessel suitable for use inpre-purification of a feed air stream in a cryogenic air separationunit;

FIG. 2 depicts a partial, cross-section view of a section of analternate embodiment of a pre-purification unit or pre-purificationvessel also suitable for use in pre-purification of a feed air stream ina cryogenic air separation unit;

FIG. 3 depicts a partial, cross-section view of a section of yet anotherembodiment of a pre-purification unit or pre-purification vessel alsosuitable for use in pre-purification of a feed air stream in a cryogenicair separation unit;

FIG. 4 is a graph that shows experimental data relating to hydrogenremoval as a function of time for certain hopcalite catalysts; and

FIG. 5 is a bar chart that shows experimental data relating to effect ofcarbon monoxide and carbon dioxide on hydrogen removal for certainpalladium based catalyst materials after an 8 hour cycle time.

DETAILED DESCRIPTION

The present system and method for pre-purification embodies a processfor removing gaseous impurities from a feed gas stream and is targetedfor applications where the purified stream is subsequently introducedinto a cryogenic distillation column such as cryogenic air separation.The disclosed pre-purification process comprises an adsorption andcatalyst based process for removing water, hydrogen, carbon monoxide andcarbon dioxide as well as other impurities from the feed stream gas.

The process comprises passing a feed stream gas containing theseimpurities through a multi-layer pre-purification vessel that ischaracterized as comprising at least three purification sectionsarranged in an adjacent manner such that the gas stream to be purifiedflows sequentially from the first purification section to the secondpurification section, and then to the third purification section alldisposed within the pre-purification vessel. It is understood that thearrangement of the three purification sections and the individual layersof materials within each section may be oriented such that the flow isin an axial orientation of the pre-purification vessel or may beoriented such that the flow is in an radial direction within thepre-purification vessel. It is also understood that pre-purificationunits may include two or more pre-purification vessels in which at leastone of the pre-purification vessels is used for pre-purification serviceremoving impurities from the feed gas stream while at least one otherpre-purification vessel is being regenerated, preferably with a purge orregeneration gas stream. The beds switch between pre-purificationservice and regeneration service periodically.

The first purification section of the pre-purifier vessel is configuredto remove impurities such as water, carbon dioxide, and optionally otherimpurities such as heavy hydrocarbons and oxides of nitrogen. The firstpurification section of the pre-purifier vessel may be comprised of amolecular sieve or one or more layers of adsorbents such as activatedalumina, silica gel or an X type zeolite such as NaX zeolite. Theindividual layers layer may also be a composite of these materials. Forremoval of hydrocarbon impurities a hydrocarbon adsorbent is oftenselected from the group consisting of types A and X zeolites and silicagel. Likewise, where removal of oxides of nitrogen are required, anadsorbent layer may include A, X, or Y type zeolites.

The second purification section of the pre-purifier vessel is configuredto remove carbon monoxide and hydrogen from the gas stream exiting thefirst purification section, with the carbon monoxide preferably removedvia catalysis and adsorption while the hydrogen generally removed bychemisorption, adsorption and catalysis. The degree to which hydrogen isremoved by catalysis or via adsorption and/or chemisorption depends onthe materials used within individual layers of the second purificationsection.

The third purification section of the pre-purifier unit is configured tofurther remove any water and carbon dioxide that exit the secondpurification section to produce a pre-purified gas stream substantiallyfree of water, carbon dioxide, carbon monoxide, hydrogen, and otherimpurities. Similar to the first purification section, the thirdpurification section may be comprised of one or more layers ofadsorbents such as activated alumina, silica gel or an X type zeolitesuch as NaX zeolite. Individual layers layer may also be a composite ofsuch materials.

As used herein, the phrase substantially free of hydrogen is a relativeterm that depends on the hydrogen content in the feed gas. For airpre-purification in a cryogenic air separation unit, substantially freeof hydrogen would typically mean less than about 500 ppb hydrogen orless than 20% of the hydrogen content in the feed gas, whicheverconcentration is lower. Likewise, the phrase substantially free ofcarbon monoxide is also a relative term that depends on the carbonmonoxide content in the feed gas and for air pre-purificationapplications typically would mean less than about 50 ppb carbon monoxideor less than 10% of the carbon monoxide content in the feed air,whichever concentration is lower. Substantially free of carbon dioxideand substantially free of water in air pre-purification applications forcryogenic air separation units are generally understood to mean aconcentration of 10 ppm or less.

The pre-purification vessel is configured to operate at the usual gasflows applicable for air separation units and well-known pressuresemployed for pre-purification of air in air separation units, generallyin the range of between about 0.2 bar(a) and about 25.0 bar(a) duringregeneration and/or purification steps. Likewise, the present system andmethod are designed to operate at temperatures that range from 5° C. to55° C. for the purification steps and temperatures as high as 200° C.for any regeneration steps. Tuning now to FIG. 1 , there is shown apre-purification unit 10 comprised of a vessel 15 configured to receivea feed gas stream at inlet 20 and deliver a purified gas stream atoutlet 60. Within the vessel 15 there are shown seven (7) layers ofmaterials used to purify the feed gas stream. These seven (7) layers arebroadly characterized herein as defining three purification sections, asdescribed below.

The first purification section 30 of the pre-purifier unit 10 isconfigured to remove impurities such as water, carbon dioxide, andoptionally other impurities such as heavy hydrocarbons and oxides ofnitrogen. The first purification section 30 of the pre-purifier unit 10includes three layers of adsorbents such as activated alumina, silicagel or an X type zeolite such as NaX zeolite or combinations thereof,including adsorbent layers 32, 34, and 36.

The second purification section 40 includes a first catalyst layer ofhopcalite 41 configured to remove at least some of the carbon monoxideand some of the hydrogen from the dry feed stream exiting adsorbentlayer 36 and entering the second purification section 40. The secondpurification section 40 of the pre-purifier unit 10 further comprises asecond layer 43 disposed downstream of the first layer 41 configured toremove water and carbon dioxide from the gas stream exiting the firstlayer 41. This second layer 43 is preferably a zeolite layer. A thirdlayer depicted as another hopcalite catalyst layer 45 is disposeddownstream of the second layer 43 and is configured to further removehydrogen and carbon monoxide from gas stream exiting second layer 43.

The third purification section 50 of the pre-purifier unit 10 isconfigured to further remove any water and carbon dioxide that exit thesecond purification section 40 to produce a pre-purified gas streamsubstantially free of water, carbon dioxide, carbon monoxide hydrogen,and other impurities. The purified gas stream exits the pre-purifierunit 10 via outlet 60. The third purification section 50 is shown as onelayer 52 of adsorbent such as activated alumina, silica gel or an X typezeolite, or mixtures thereof.

A plurality of flat separation screens 70 are preferably installed flushto the vessel wall between the various hopcalite catalyst layers 41, 45and the adjacent adsorbent layers 36, 43, 52. The separation screens arepreferably made of Monel due to presence of high oxygen content in theregeneration gas.

In the alternate embodiment shown in FIG. 2 , there is apre-purification unit 100 comprised of a vessel 115 configured toreceive a feed gas stream at inlet 120 and deliver a purified gas streamat outlet 160. Within the vessel 115 there are shown nine (9) layers ofmaterials used to purify the feed gas stream, divided generally intothree purification sections. The first purification section 130 of thepre-purifier unit 100 is configured to remove impurities such as water,carbon dioxide, and optionally other impurities such as hydrocarbons andoxides of nitrogen in multiple layers 132, 134, and 136 of adsorbentmaterials, such as activated alumina, silica gel or an X type zeolitesuch as NaX zeolite or combinations thereof.

The second purification section 140 includes a first hopcalite catalystlayer 141 configured to remove at least some of the carbon monoxide andsome of the hydrogen from the dry feed stream exiting adsorbent layer136 and entering the second purification section 140. The secondpurification section 140 of the pre-purifier unit 100 further comprisesan adsorbent layer 143 disposed downstream of the first hopcalite layer141 configured to remove water and carbon dioxide from the gas streamexiting the first hopcalite layer 141. This adsorbent layer 143 ispreferably a zeolite layer. Another hopcalite catalyst layer 145 isdisposed downstream of the adsorbent layer 143 and is configured tofurther remove hydrogen and carbon monoxide from gas stream exiting thesecond layer 143. Another adsorbent layer 147 configured to remove waterand carbon dioxide from the gas stream exiting the second hopcalitelayer 145 is disposed downstream of the second hopcalite layer 145.Finally, a third hopcalite layer 149 configured to remove substantiallyall of the remaining hydrogen is disposed downstream of the adsorbentlayer 147. Similar to the embodiment of FIG. 1 , a plurality of Monelseparation screens 170 are preferably installed between the varioushopcalite catalyst layers 141, 145, 149 and the adjacent adsorbentlayers 136, 143, 147, 152.

The third purification section 150 of the pre-purifier unit 100 is shownas one layer 152 of adsorbent such as activated alumina, silica gel oran X type zeolite, or mixtures thereof and is configured to furtherremove any water and carbon dioxide that exit the second purificationsection 140 to produce a pre-purified gas stream substantially free ofwater, carbon dioxide, carbon monoxide hydrogen, and other impurities.

In both embodiments depicted in FIGS. 1 and 2 , the second purificationsections 40, 140 of the pre-purifier units 10, 100 may be broadlycharacterized as having two or more separate layers of hopcalite withthe successive layers of the hopcalite separated by a zeolite adsorbentlayer that removes water and carbon dioxide produced in the hopcalitelayers.

Tuning now to FIG. 3 , there is shown yet another embodiment of thepresent system and method for pre-purification of a feed gas stream thatincludes a vessel 215 configured to receive a feed gas stream at inlet220 and deliver a purified gas stream at outlet 260. In the embodimentshown in FIG. 3 , the first purification section 230 includes a layer ofalumina 233 and a layer of zeolite based molecular sieve 235 while thethird purification section 250 of the pre-purifier unit 200 isconfigured with a capping layer 252 of zeolite based molecular sieve toremove impurities such as water, carbon dioxide exiting the secondpurification section 240. As with the embodiments shown and describedwith reference to FIGS. 1 and 2 , the various layers of adsorbentmaterials in the first and third purification sections may be activatedalumina, silica gel or an X type zeolites or combinations thereof toremove impurities such as water, carbon dioxide, and optionally otherimpurities in the gas streams flowing through such layers.

The second purification section 240 of the pre-purifier unit 200, on theother hand includes a hopcalite catalyst layer 241 configured to removemost of the carbon monoxide and some of the hydrogen from the dry gasstream exiting adsorbent layer 236 and entering the second purificationsection 240 followed by an adsorbent layer 243 disposed downstream ofthe hopcalite layer 241 configured to remove water and carbon dioxidefrom the gas stream exiting the hopcalite layer 241. This adsorbentlayer 243 is preferably a zeolite based molecular sieve. Different fromthe embodiments shown and described with reference to FIGS. 1 and 2 ,the embodiment shown in FIG. 3 includes a layer of noble metal catalystsuch as 0.5 wt % Pd/Al₂O₃ instead of additional hopcalite catalystlayers. In the layer of noble metal catalyst 244, and hydrogen from theintermediate gas stream exiting adsorbent layer 243 that has beencleansed of carbon dioxide and water is partially oxidized to waterand/or adsorbed by this catalyst layer 244 while any residual carbonmonoxide escaping the upstream hopcalite layer may also be oxidized inthis layer. Much of the water produced in the partial oxidation ofhydrogen may be adsorbed in the Al₂O₃ catalyst support while some of thewater and carbon dioxide produced will continue to the thirdpurification section 250. A plurality of Monel separation screens 270are preferably installed between the various hopcalite catalyst layers241 or catalyst layer 244 and the adjacent adsorbent layers 236, 243,252.

As is well known in the art, air pre-purification systems use two ormore pre-purification units or vessels so as to allow continuousproduction of purified air. When one or more of the pre-purificationunits is purifying the feed air, one or more other pre-purificationunits are being regenerated, preferably using a process widely known asthermal regeneration. The thermal regeneration process acts to desorbsthe water and carbon dioxide from various layers in the pre-purifierunits while also restoring the hydrogen adsorption capacity of thehopcalite catalyst layers and other catalyst layers.

Thermal regeneration is preferably done using a multi-step process thatoften involves the following four steps: (i) depressurizing the vesselto lower pressures suitable for regeneration; (ii) heating the layerswithin the vessel to desorb the water and carbon dioxide from variouslayers and restore the hydrogen adsorption capacity of the hopcalitecatalyst layers and/or other catalyst layers; (iii) cooling the layerswithin the vessel back to temperatures suitable for the purificationprocess; and (iv) repressurizing the vessel back to the higher operatingpressures required for the purification process. While thermalregeneration is preferred, it is contemplated that the present systemand methods could be used with pressure swing adsorption basedpre-purifiers or even hybrid type pre-purifiers.

Thermal regeneration is preferably conducted at lower pressures such as1.0 to 1.5 bar(a) compared to the purification process and must beconducted at temperatures of at least 180° C., and more preferably attemperatures of about 190° C., or more, subject to appropriate safetyrequirements. The heating step in the thermal regeneration process istypically conducted by heating a purge gas to produce a stream of hotpurge gas which is fed to vessel via outlet 60, 160, 260 and whichtraverses the layers of the pre-purifier unit 10, 100, 200 in reverseorder compared to the above-described purification process. In manyapplications, the purge gas may be taken as a portion of the product gasor from waste gas from the distillation columns of the cryogenic airseparation unit. As the hot purge gas passes through the varioussections and layers of the pre-purifier unit 10, 100, 200, the catalystlayers and adsorbent layers are regenerated. The effluent purge gasexiting the pre-purification unit 10, 100, 200 via the inlet 20, 120,220 is typically vented. After the catalyst layers and adsorbent layersare heated and regenerated, the pre-purification unit is then cooledusing a cool purge gas generally at a temperature from about 10° C. upto 50° C. that flows through the pre-purification unit in the samedirection as the hot purge gas. After cooling, the vessel isrepressurized to the higher operating pressures required by thepurification process.

The regeneration steps are conducted as described for a predeterminedperiod of time, typically referred to as the cycle time after which theservice or functions of the pre-purification units are switched so thatvessels previously regenerating come “on line’ and initiates thepurification process while vessels previously purifying the feed air go“off-line’ and initiate the regeneration process. Typicalpre-purification cycle times for high-purity or ultra-high puritynitrogen producing air separation plants is between about 240 minutesand 480 minutes. In this manner, each pre-purification unit alternatesbetween purification service and regeneration service to maintaincontinuous production of purified air substantially free of carbondioxide, water, carbon monoxide, hydrogen and other impurities.

The pre-purifier vessels depicted in FIGS. 1-3 . are preferably denseloaded. Dense loading provides the most consistent and uniform packingof adsorbents and catalysts with minimal leveling of the layersrequired. Furthermore, dense packing minimizes adsorbent settling. Suchdense packing for pre-purifiers designed for carbon monoxide andhydrogen removal is optional and may be utilized for all layers in thebed to ensure integrity and uniform depth. Because of relatively thinlayers in second purification section for removal of carbon monoxide andhydrogen, including multiple layers of hopcalite and adsorbent layers,as well as any noble metal based catalyst that may be used, it isimportant to minimize the shifting and/or settling of the layers inorder to maintain a uniform depth of the layers over the life of thepre-purifier unit.

Example 1

The embodiment of FIG. 1 has been evaluated using computer basedsimulations and models to demonstrate the expected performance of thepre-purification unit and Table 1 shows modeling data for the gasstreams entering the pre-purifier and exiting each individual layer ofmaterial in the pre-purifier unit. The flow rate through thepre-purifier is modeled at about 99000 Nm³/h for a cycle time of between2 to 6 hours. For sake of brevity and simplicity, the followingdiscussion focuses on the second purification section of thepre-purifier unit in an effort to demonstrate performance and costbenefits of this arrangement compared to prior art pre-purificationsystems.

TABLE 1 Impurities (at Exit of Layer) PPU Layer Material Length CO(ppb)H₂(ppb) H₂O(ppm) CO₂(ppm) PPU Inlet — — 1000 1000 2000 450 Section 1 -Layer 1 Alumina 10 cm 1000 1000 2000 450 Section 1 - Layer 2 Alumina 40cm 1000 1000 10 450 Section 1 - Layer 3 Zeolite 100 cm  1000 1000 <0.01<0.1 Section 2 - Layer 4 Hopcalite 18 cm <1 100 <1.0 1.0 Section 2 -Layer 5 Zeolite 20 cm <1 100 <0.01 <0.01 Section 2 - Layer 6 Hopcalite10 cm <0.1 5 <0.1 <0.1 Section 3 - Layer 7 Zeolite 20 cm <0.1 5 <0.01<0.01 PPU Outlet — — <0.1 5 <0.01 <0.01

With reference to the data in Table 1 for this two-layer hopcalite basedarrangement, the initial hopcalite layer or first catalyst layer isabout 18 cm in length and configured to remove most of the carbonmonoxide via an oxidation with the copper and magnesium oxides in thecatalyst layer to produce carbon dioxide most of which may be adsorbedin the hopcalite layer and some of which exits the first catalyst layer.Concurrently, a first portion of the hydrogen in the gas streamtraversing the first catalyst layer is oxidized to produce water while asecond portion of the hydrogen in the gas stream traversing the firstcatalyst layer is adsorbed in the first catalyst layer while a thirdportion of the hydrogen impurities in the gas stream traversing thefirst catalyst layer passes through the first catalyst layer. Thehydrogen and carbon monoxide profiles depicted in Table 1 shows adistinct increase in water impurities and carbon dioxide in the gasstream exiting the first catalyst layer (i.e. not absorbed in the firstcatalyst layer), presumably from oxidation of the hydrogen and carbonmonoxide, respectively. The hydrogen reduction from about 1000 ppb to100 ppb is a net reduction of about 90% while the carbon monoxide showsa net reduction of about 99.9% from about 1000 ppb to about 1.0 ppb.Note that the hydrogen impurities are being removed in this firsthopcalite layer at an average rate of 5.0% per cm of hopcalite.

The second layer in the second section of the pre-purifier unit is azeolite based adsorbent about 20 cm in length that removes the waterimpurities from about 1.0 ppm to about 0.01 ppm and also removes carbondioxide from about 1.0 ppm to less than about 0.01 ppm.

The third layer in the second section of the pre-purifier unit is asecond hopcalite layer about 10 cm in length that receives the gasstream cleansed of water and carbon dioxide exiting the second layer andis configured to further remove the hydrogen from about 100 ppm to about5 ppm for a reduction of about 95% of the remaining hydrogen and removesany remaining carbon monoxide to levels below 0.1 ppb resulting in a gassubstantially free of carbon monoxide. Put another way, in this thirdlayer the hydrogen impurities are being removed at an average rate of9.5% per cm of hopcalite compared to an average rate of hydrogen removalof 5.0% per cm of hopcalite in the first layer of hopcalite catalyst.Again, the removal of hydrogen and substantially all of the carbonmonoxide yields an exit gas with less than about 0.1 ppm water and lessthan about 0.1 ppm of carbon dioxide, with much of the produced waterand carbon monoxide being adsorbed in the third layer (i.e. secondhopcalite layer).

Example 2

The embodiment of FIG. 2 has also been evaluated using computer basedsimulations and models to also demonstrate the expected performance ofthe depicted multi-layer hopcalite based pre-purification unit and Table2 shows modeling data for the gas streams entering the pre-purifiershown and described with reference to FIG. 2 . As with the discussionabove regarding the data in Table 1, the materials are the same as inthe previous example and the flow rate of the air through thepre-purifier is also modeled at 99000 Nm³/h for a cycle time of between2 hours and 6 hours.

TABLE 2 Impurities (at Exit of Layer) PPU Layer Material Length CO(ppb)H₂(ppb) H₂O(ppm) CO₂(ppm) PPU Inlet — — 1000 1000 2000 450 Section 1 -Layer 1 Alumina 10 cm 1000 1000 2000 450 Section 1 - Layer 2 Alumina 40cm 1000 1000 10 450 Section 1 - Layer 3 Zeolite 100 cm  1000 1000 <0.01<0.1 Section 2 - Layer 4 Hopcalite 10 cm <1 321 <0.7 1.0 Section 2 -Layer 5 Zeolite 20 cm <1 321 <0.01 <0.01 Section 2 -Layer 6 Hopcalite 10cm <0.1 60 <0.3 <0.1 Section 2 -Layer 7 Zeolite 10 cm <0.1 60 <0.01<0.01 Section 2 -Layer 8 Hopcalite  8 cm <0.1 5 <0.05 <0.01 Section 3-Layer 9 Zeolite 10 cm <0.1 5 <0.01 <0.01 PPU Outlet — — <0.1 5 <0.01<0.01

With reference to the data in Table 2 and focusing on the second sectionof the pre-purifier unit which comprises a multi-layer hopcalite basedarrangement, the initial hopcalite layer or first catalyst layer in thesecond purification section, identified as layer 4 in Table 2, is about10 cm in length and configured to remove most of the carbon monoxide viaan oxidation in the catalyst layer to produce carbon dioxide most ofwhich may be adsorbed in hopcalite layer and some of which exits thefirst catalyst layer. Concurrently, a first portion of the hydrogen inthe gas stream traversing the first catalyst layer is oxidized toproduce water while a second portion of the hydrogen in the gas streamtraversing the first catalyst layer is adsorbed while a third portion ofthe hydrogen impurities in the gas stream traversing the first catalystlayer passes through the first catalyst layer. The hydrogen and carbonmonoxide profiles depicted in Table 2 shows a distinct increase in waterimpurities and carbon dioxide in the gas stream exiting the firstcatalyst layer (i.e. impurities not absorbed in the first catalystlayer), presumably produced from oxidation of hydrogen and carbonmonoxide, respectively. The hydrogen reduction from about 1000 ppb to321 ppb is a net reduction of only about 68% while the carbon monoxideshows a net reduction of about 99.9% from 1000 ppb to about 1.0 ppb.

The second layer in the second section of the pre-purifier unitidentified as layer 5 is a zeolite based adsorbent about 20 cm in lengththat removes the water impurities from about 0.7 ppm to about 0.01 ppmand removes carbon dioxide from about 1.0 ppm to less than 0.01 ppm.

The third layer in the second section of the pre-purifier unitidentified as layer 6 is another hopcalite layer about 10 cm in lengththat receives the gas stream cleansed of water and carbon dioxideexiting the second layer and is configured to further remove thehydrogen from about 321 ppm to about 60 ppm for a reduction of about 81%of the remaining hydrogen and removes any remaining carbon monoxide tolevels below 0.1 ppb resulting in a gas substantially free of carbonmonoxide. Again, the removal of hydrogen and substantially all of thecarbon monoxide yields an exit gas with about 0.3 ppm water and up toabout 0.1 ppm of carbon dioxide, with much of the produced water andcarbon monoxide being adsorbed in the third layer (i.e. second hopcalitelayer).

The fourth layer in the second section of the pre-purifier unitidentified as layer 7 is another zeolite based adsorbent that againremoves the water impurities from about 0.3 ppm back down to about 0.01ppm level and removes carbon dioxide from about 0.1 ppm to less than0.01 ppm levels. This fourth layer of zeolite based adsorbent is onlyabout 10 cm in length, thus helping reduce cost of materials.

The fifth layer in the second section of the pre-purifier unit isidentified as layer 7 and is yet another hopcalite layer of only about8.0 cm in length that receives the gas stream exiting the fourth layerand is configured to further remove most of the remaining hydrogenimpurities from about 60 ppb level to about 5 ppb level for a reductionof about 92% of the remaining hydrogen to yield an effluent gas that issubstantially free of both hydrogen and carbon monoxide.

Advantageously, by using this multi-layer arrangement with multiplelayers of hopcalite separated by intermediate layers of an adsorbentconfigured to remove water and carbon dioxide, there is a noticeableimprovement in hydrogen removal capacity. In the modeled arrangement,the first hopcalite layer is 10 cm in length and removes 68% of thehydrogen in the stream traversing that first hopcalite layer whereas thesecond hopcalite layer is also 10 cm in length yet removes 81% of thehydrogen in the stream traversing that second hopcalite layer. The thirdhopcalite layer is only about 8 cm in length yet removes about 92% ofthe hydrogen in the stream traversing that third hopcalite layer. Inthis manner, hydrogen removal is performed in a cascading manner wherethe efficiency of hydrogen removal improves in successive hopcalitelayers.

Without being bound by any particular theory or design limitations,using a multi-layer, hopcalite based pre-purifier design with thiscascading hydrogen removal, one can improve hydrogen removal bydesigning the first hopcalite layer to remove between 50% and less than90% of the hydrogen in the feed stream, and in some embodiments removebetween 50% and less than 75% of the hydrogen in the feed stream. Thelast hopcalite layer is preferably configured to remove more than 90% ofthe hydrogen entering the last hopcalite layer. Intermediate hopcalitelayers, if used, are preferably designed or configured to removerelatively more hydrogen than the preceding hopcalite layer, measured asa percentage of hydrogen in the gas stream entering the hopcalite layer.Intermediate hopcalite layers can be preferably configured to removebetween 51% and 89% of the hydrogen entering that intermediate hopcalitelayer.

Example 3

FIG. 4 shows a graph of data obtained from a plurality of laboratorytests showing the hydrogen removal characteristics of a hopcalitecatalyst, specifically a 22.86 cm long bed of Carulite®. The laboratorytests passed air at a temperature of 20° C. (i.e. curve 301) or 40′ C(i.e. curves 302 and 303), a pressure of about 9.6 bar(a), and a flowrate of 13.3 slpm. The feed air stream had 3 ppm of hydrogen and either10 ppm of carbon monoxide (i.e. curves 301 and 302) or 1 ppm of carbonmonoxide (i.e. curve 303).

As seen in FIG. 4 , the ratio of hydrogen concentration exiting the bedof Carulite catalyst to the hydrogen concentration entering the bed ofCarulite catalyst as a function of time is shown for three multipledifferent conditions. Curve 301 represents test conditions with the airstream at 20° C., the hydrogen level at the inlet of 3 ppm and thecarbon monoxide level at the inlet of 10 ppm and shows a hydrogen ratioof about 0.7 after 100 minutes and about 0.85 after 350 minutes.Increasing the temperature to 40° C., while keeping the 3 ppm hydrogenlevel at the inlet and 10 ppm carbon monoxide level at the inletimproves the hydrogen removal performance as shown in curve 302.Specifically, curve 302 shows a hydrogen ratio of about 0.4 after 100minutes and about 0.7 after 350 minutes. Curve 303 shows even betterhydrogen removal performance with the feed air temperature at 40° C. butreducing the carbon monoxide concentration in the feed air to 1 ppmwhile keeping the 3 ppm hydrogen impurity level at the inlet.Specifically, curve 303 shows a hydrogen ratio of about 0.3 after 100minutes and about 0.5 after 350 minutes. The improved hydrogen reductionbetween curves 302 and 303 suggests that the hydrogen removalperformance in a second and/or third layers of hopcalite 45, 145, 149 inthe embodiments shown in FIGS. 1 and/or 2 will be improved because thecarbon monoxide impurity levels entering the second and/or third layersof hopcalite is less than 1 ppm as shown in Tables 1 and 2.

Example 4

FIG. 5 shows a graph of data obtained from a plurality of laboratorytests showing the hydrogen removal characteristics of a 22.86 cm longbed of a palladium based catalyst such as 0.5 wt % Pd/Al₂O₃. In thisexample, synthetic air at a pressure of about 11 bar(a), a temperatureof 10° C. is passed through a tube containing the palladium basedcatalyst for a cycle time of about 480 minutes. The feed air stream had3 ppm of hydrogen impurities and either 1 ppm of carbon monoxideimpurities (i.e. bar 401) or 1 ppm of carbon dioxide (i.e. bar 402) orno carbon monoxide and no carbon dioxide impurities (i.e. curve 403).The height of each bar is representative of the average hydrogenbreakthrough at the end of the 480 minute cycle across multiple tests.

As seen in FIG. 5 , the average hydrogen concentration exiting the tubeof catalyst at the end of the 480 minute cycle when the feed air has 3ppm hydrogen and 1 ppm carbon monoxide is over 50 ppb (i.e. bar 401).Comparatively, the average hydrogen concentration exiting the tube ofcatalyst at the end of the 480 minute cycle when the feed air has 3 ppmhydrogen and 1 ppm carbon dioxide is just over 20 ppb, suggestinghydrogen removal in the catalyst is improved if the treated stream hasless carbon monoxide. However, the average hydrogen concentrationexiting the tube of catalyst at the end of the 480 minute cycle when thefeed air has 3 ppm hydrogen and little or no carbon dioxide or carbondioxide is less than 5 ppb, suggesting hydrogen removal in the catalystis improved if the treated stream has substantially no carbon dioxideand no carbon monoxide.

The improved hydrogen reduction shown in bar 403 compared to hydrogenreduction depicted by curves 401 and 402 further suggests that thehydrogen removal performance in a palladium based catalyst layer 244 inthe embodiment of FIG. 3 will be improved by the presence of adsorbentlayer 243 which removes carbon dioxide and water before the gas streamenters the palladium catalyst layer. In addition, this data alsosuggests that the hydrogen removal performance in the second and/orthird layers 45, 145, 149 of hopcalite in the embodiments shown in FIGS.1 and/or 2 will be improved because of the prior hopcalite layers andadjacent adsorbent layers causing the gas stream entering the secondand/or third layer of hopcalite layers to be substantially free ofcarbon monoxide and carbon dioxide as shown in Tables 1 and 2.

While the present system and method have been described with referenceto a preferred embodiment or embodiments, it is understood that numerousadditions, changes and omissions can be made without departing from thespirit and scope of the present invention as set forth in the appendedclaims.

What is claimed is:
 1. A pre-purifier unit for an air-separation unitcomprising: a vessel having an inlet configured for receiving a streamof compressed feed air and an outlet configured to direct a purifiedstream substantially free of water, carbon dioxide, carbon monoxide andhydrogen to another part of the air separation unit; a firstpurification section disposed within the vessel comprising at least onelayer of adsorbent configured to remove water and carbon dioxide fromthe compressed feed air stream and yield a dry feed stream substantiallyfree of water and carbon dioxide; a second purification section disposedwithin the vessel downstream of the first purification section, thesecond purification section comprising (i) a first layer of manganeseoxide and copper oxide containing catalyst configured to remove at leastsome of the carbon monoxide and hydrogen from the dry feed stream andproduce a first intermediate effluent, (ii) a second layer disposeddownstream of the first layer configured to remove water and carbondioxide from the intermediate effluent and produce a second intermediateeffluent, and (iii) a third manganese oxide and copper oxide containingcatalyst layer disposed downstream of the second layer and is configuredto remove hydrogen from the second intermediate effluent to yield anintermediate purified stream substantially free of hydrogen and carbonmonoxide; and a third purification section disposed within the vesseldownstream of the second purification section and comprising at leastone other layer of adsorbent configured to remove water and carbondioxide from the intermediate purified stream and yield the purifiedstream substantially free of water, carbon dioxide, carbon monoxide andhydrogen.
 2. The pre-purifier unit of claim 1, wherein the secondpurification section further comprises a fourth layer disposeddownstream of the third manganese oxide and copper oxide containingcatalyst layer, wherein the fourth layer is a noble metal containingcatalyst layer.
 3. The pre-purifier unit of claim 2, wherein the fourthlayer is a palladium containing polishing layer.
 4. The pre-purifierunit of claim 1, wherein the second purification section furthercomprises one or more additional layers of manganese oxide and copperoxide containing catalyst configured to remove carbon monoxide andhydrogen and one or more additional adsorbent layers configured toremove water and carbon dioxide.
 5. The pre-purifier unit of claim 1,wherein the dry feed stream is substantially free of hydrocarbons andnitrous oxide.
 6. The pre-purifier unit of claim 1, wherein the secondlayer comprises a molecular sieve layer or a layer of alumina or both amolecular sieve layer and a layer of alumina.
 7. The pre-purifier unitof claim 1, wherein the at least one adsorbent layer in the firstpurification section comprises a molecular sieve layer or a layer ofalumina or both a molecular sieve layer and a layer of alumina.
 8. Thepre-purifier unit of claim 1, wherein the at least one other adsorbentlayer in the third purification section comprises a molecular sievelayer or a layer of alumina or both a molecular sieve layer and a layerof alumina.
 9. The pre-purifier unit of claim 1, wherein thepre-purifier unit is configured to purify the compressed feed air streamthat contains less than 20 ppm hydrogen.
 10. The pre-purifier unit ofclaim 1, wherein the pre-purifier unit is configured to purify thecompressed feed air stream that contains less than less than 50 ppmcarbon monoxide.
 11. The pre-purifier unit of claim 1, wherein the dryfeed stream contains less than 10 ppm water.
 12. The pre-purifier unitof claim 1, wherein the dry feed stream contains less than 10 ppm carbondioxide.
 13. The pre-purifier unit of claim 1, wherein the secondintermediate effluent contains no more than 10 ppm water.
 14. Thepre-purifier unit of claim 1, wherein the second intermediate effluentcontains no more than 10 ppm carbon dioxide.
 15. The pre-purifier unitof claim 1, wherein the purified stream comprises between 10 ppbhydrogen and 500 ppb hydrogen.
 16. The pre-purifier unit of claim 1,wherein the purified stream comprises no more than 10 ppb hydrogen. 17.The pre-purifier unit of claim 1, wherein the purified stream comprisesno more than 10 ppb carbon monoxide.
 18. The pre-purifier unit of claim1, wherein the pre-purifier unit is configured to purify the compressedfeed air stream that is at a pressure between 3 bar(a) and 30 bar(a).19. The pre-purifier unit of claim 1, wherein the pre-purifier unit isconfigured to purify the compressed feed air stream that is at atemperature between 0° C. and 70° C.
 20. The pre-purifier unit of claim1, wherein the pre-purifier unit further comprises at least two vesselseach having the first purification section, second purification sectionand third purification section and the pre-purifier unit is configuredas a thermal swing adsorption based pre-purifier.
 21. The pre-purifierunit of claim 1, wherein the pre-purifier unit further comprises atleast two vessels each having the first purification section, secondpurification section and third purification section and the pre-purifierunit is configured as a pressure swing adsorption based pre-purifier.22. The pre-purifier unit of claim 1, wherein the pre-purifier unitfurther comprises at least two vessels each having the firstpurification section, second purification section and third purificationsection and the pre-purifier unit is configured as a hybrid thermalswing adsorption and pressure swing adsorption pre-purifier.